The drugmaker's shares, down 21 percent this year, fell about 2 percent in mid-day trade on Thursday.

Regeneron is bracing for a hit to Eylea's sales by focusing on expanding Dupixent's label to reach more patients and venturing into the hot new field of immuno-oncology with a potential regulatory approval later this year.

"There are concerns that what happened (with Dupixent) in first quarter could happen again," SunTrust Robinson Humphrey analyst Yatin Suneja said, adding that there could possibly be an issue with the long term growth potential of the drug.

Dupixent's quarterly sales of $131 million, reported last week by partner Sanofi, missed estimates of $167.6 million, a worrying sign that the expensive drug might be facing similar issues like the company's pricey drug Praluent.

Just two days ago, Regeneron and partner Sanofi SA slashed the price on Praluent by more than half for Express Scripts customers, an aggressive attempt to boost sales of the drug.

Praluent earned $60 million in first-quarter revenue, falling short of expectations of $66 million, according to Thomson Reuters I/B/E/S.

One bright spot for Regeneron was Eylea sales, which rose 15 percent to $984 million, beating estimates of $922.4 million.

While Eylea has driven much of Regeneron's growth for more than seven years, competition from Lucentis, a rival treatment from Roche AG and Novartis AG, remain a worry for analysts and investors.

"There's even more competition on the horizon for Eylea, so we are looking at Dupixent as a growth driver," RBC Capital Markets analyst Kennen MacKay said.

The company said it is unlikely to be active in dealmaking and instead will rely on its own drug pipeline even as large drugmakers across the world spend big to acquire nimbler, smaller rivals.

"Our strategy was, and continues to be, to invest heavily in our internal research capabilities rather than look externally for new products," Chief Executive Officer Leonard Schleifer said on a conference call with analysts.

"We are not nearly as desperate as other companies are to fill up gaps in the pipeline."

Regeneron's net income rose 92 percent to $478 million.

Excluding one-time items, the company earned $4.67 per share, ahead of estimates of $4.35.